Jeffrey Wilkes: SuperKamiokande
Wilkes gave a review of the fantastic amount of information that was extracted from many years of data taking by the SuperKamiokande detector in the course of the last 15 years. I took sparse notes during his talk, but his slides are already available on the conference site. Here let me just give a short overview.
The present instantiation of SuperKamiokande (see right for a sketch of the detector) is version 4. The original one dates back to 1996, and is the one which produced the ground-breaking observation of atmospheric neutrino oscillations. The photomultiplier coverage back then was of 40%.
Version 2 came in four years later, when many dead PMT were replaced, and during the refilling of water in the tank occurred the famous implosion accident. They spent some time rebuiliding the detector, transferring surviving PMT into alternate positions, and finally achieving a 40% coverage again. The improvements had significant effects in the energy threshold achievable for solar neutrinos, but the improvement was less significant for atmospheric neutrino physics.
Then came SKIII, which still had the original electronics, but enhanced PMT coverage. In 2008 the detector underwent another upgrade, where all the vintage front-end electronics was replaced it with contemporary hardware.
Wilkes showed one of the landmark results of SK: the high-statistics results for the fluxes of atmospheric neutrino as a function of zenith angles, which shows clearly the oscillation effect.
He then showed the current results of a two-flavour oscillation analysis on the delta m^2 – sin^2 (2 theta) plane, overlaid with the recent MINOS result. The results of the combined analysis give a consistent picture. Notably, these disfavour sterile neutrino disappearance at 7 sigma significance.
For the full 3-flavour oscillation results, they consider matter effects and a “solar term” simultaneously. The latter is a possible enhancement of electron neutrinos in the sub-GeV region. The 2-flavour and full-flavour analysis results are consistent; he observed that there is no significant change in the best-fit sin^2 (2 theta_23).
An interesting result -one which I had not been aware of, at least- was then shown by the speaker. Tau neutrino events can be searched in SuperKamiokande data. These interactions give rise a complicated event topology, which makes the identification of the leading lepton from tau decay difficult. They use a neural network, and since they expect a negligible primary tau flux, the goal is to detect tau neutrinos in atmospheric data, to test the “no tau appearance” hypothesis. This results in an excess from tau neutrinos (see figure on the left), a signal of over 200 tau-neutrino events. SuperKamiokande data are inconsistent with the “no tau appearance” at 3.8 standard deviations.
Another analysis seeks evidence for CPT vilation phenomena in atmospheric neutrino data. They test whether neutrino and antineutrino mixing parameters are indeed the same. This is confirmed by the existing data.
They search also for rare processes. For nucleon decays, the lifetime of protons in the decay into pion-antineutrino pairs is >3.9E32 years, while for the decay of neutron to antineutrino- pizero is >1.1 E33 years. These limits are starting to challenge the predictions of the SO(10) model, as is clear from the figure below, which shows the experimental limits for several decay modes (in years, on the x axis and shown by red bars) compared with theoretical predictions in blue.
For solar neutrinos, the method of detection by SuperKamiokande involves the fit of the Cherenkov rings that arise when relativistic electrons materialize in the detector. The energy resolution is not different for SKIII from what it was in SKI. From 548 days of live time, and an energy threshold of 6.5 MeV, they measure the Boron-8 flux, and the day-night ratio of -0.056+-0.031+-0.013.